In rotating compressors, and also particularly in high-pressure compressors such as are used, for example, in stationary gas turbines or turbine driving systems for the compression of combustion air, rings of rotor blades are arranged one behind the other in several pressure stages on a rotor shaft and are concentrically surrounded by a compressor casing. A radial clearance of the order of value of 1 mm is provided between the outer ends of the rotor blades and the inner wall of the compressor casing. This clearance should be kept as small as possible in order to restrict the reverse flow of air and the associated reduction in efficiency. The same applies to the rings of guide vanes which are arranged between the pressure stages and are fastened to the inner wall of the compressor casing.
The reduction in the radial clearance is made more difficult by the fact that the compressor rotor blades and the compressor casing expand and contract to different extents in the different operating conditions. The radial clearance must therefore be selected in such a way that it is still sufficient under the most unfavorable operating conditions, i.e. with an expanded rotor and rotor blades and a contracted compressor casing. Account should be taken of the fact that the change in the radial clearance can have both mechanical and thermal causes. The main mechanical cause to be considered is the radial deflection of the rotor and the rotor blades due to the centrifugal forces acting during rapid rotation. Different thermal expansions in the rotor and stator due to temperature differences or different expansion coefficients of the materials used may be regarded as the thermal causes.
A large number of proposals has been made in the past concerning active correction of the radial clearance (so called "active clearance control") during operation. In the publication mentioned at the beginning, for example, optional colder and/or warmer compressed air originating from different compressor stages is, for this purpose, directed into the rotor in order to control the radial clearance by controlling the temperature of the disks supporting the rotor blades. A comparable solution is also published in EP-B1-0 140 818. Special methods for the open-chain and closed-loop control of the clearance can, for example, be taken from U.S. Pat. No. 4,849,895.
In addition to the rotor temperature control system mentioned above, a compressor casing temperature control system has also been previously proposed (U.S. Pat. No. 4,230,436). In this system, the temperature of the compressor casing is lowered in a controlled manner by a cooling airflow of greater or lesser strength. The cooling air is taken from different compressor stages and is fed along cooling passages both behind the guide vanes and behind the inner wall of the compressor casing opposite to the rotor blades.
The known methods of active clearance control relate to normal operation of the compressor. They can therefore make use of compressor air of varying temperature or--in the case of the compressor of a gas turbine--of hot gas from the driving system part for cooling or heating different compressor parts or compressor sections.
The so-called "warm start" case is not allowed for in these arrangements. In a warm start, the compressor is started again after being previously shut down but before it has completely cooled. In this case, the rotor and the stator are at markedly different temperatures because the outer stator cools more rapidly and correspondingly contracts whereas the rotor remains hot longer and correspondingly retains its expansion. Because of this, the radial clearance is substantially reduced. In order to make renewed starting possible in this condition (warm start), this special case must be taken into account in the design of the radial clearance and this leads to increased radial clearance figures.